Field of the Invention
The present invention relates to a variable compression ratio apparatus and, more particularly, to a variable compression ratio apparatus in which a compression ratio of a mixture in a combustion chamber is varied according to an operational state of an engine.
Description of Related Art
Generally, heat efficiency of a heat engine is increased when a compression ratio is high, and in case of a spark ignition engine, when an ignition time is advanced to a certain level, heat efficiency is increased. But, when an ignition time of a spark ignition engine is advanced at a high compression ratio, abnormal combustion occurs to damage the engine, so there is a limitation in advancing an ignition time and a corresponding degradation of an output should be tolerated.
A variable compression ratio (VCR) apparatus is an apparatus for changing a compression ratio of a mixture according to an operational state of an engine. According to the VCR apparatus, a compression ratio of a mixture is increased in a low load condition to enhance mileage (or fuel efficiency), and the compression ratio of the mixture is lowered in a high load condition to prevent a generation of knocking and enhance an engine output.
The related art VCR apparatus implements a change in a compression ratio by changing a length of a connecting rod connecting a piston and a crank shaft. In the VCR apparatus, the part connecting the piston and the crack shaft includes several links, directly transmitting combustion pressure to the links. Thus, durability of the links weakens.
Various experimentation results with respect to the related art VCR apparatus revealed that operation reliability is high when a distance between the rank pin and the piston pin is changed by using an eccentric bearing. Meanwhile, when hydraulic pressure is used to rotate an eccentric cam, an amount of rotation and an amount of hydraulic outflow of the eccentric cam of each cylinder are different, result in problems in that a compression ratio of each cylinder is not uniform and a time during which a compression ratio is changed varies according to engine operational conditions.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.